The Single-Well Chemical Tracers Test (SWCTT) is a method well known in the art useful for measuring fluid saturations in oil reservoirs, for instance Residual Oil Saturation (SOR). The Residual Oil Saturation is a value representing the quantity of oil that remains in an oil reservoir at depletion. Knowing the Residual Oil Saturation of an oil reservoir may be important if one wants to carry out, for example, an Enhanced Oil Recovery method.
SWCTT has been disclosed in the U.S. Pat. No. 3,623,842. The principle of this method is based on the distribution of tracer molecules locally according to an equilibrium relationship between pockets of trapped oil and flowing water. The test method consists in injecting to the oil reservoir an aqueous solution containing a tracer. Said tracer is capable of reacting in the reservoir in order to form a secondary tracer, said first tracer and said secondary tracer having different partition coefficients between oil and water. The tracer solution is thus left in the reservoir for a reaction period. After that, the well is produced, and the returning water is analyzed. Because of the difference between the partition coefficient of the first tracer and the partition coefficient of the secondary tracer, both tracers do not reappear at the same time in the returning water. The Residual Oil Saturation SOR may be calculated from the separation between the first tracer and the secondary tracer in the returning water.
Since this first disclosure, SWCTTs have been run in several fields, and some improvements have been disclosed. For example, the U.S. Pat. No. 4,168,746 and U.S. Pat. No. 6,321,595 disclose the injection of several tracers in the oil reservoir. A thorough report performed by the U.S. Department of Energy (DOE), entitled “Fossil energy, The Single-Well Chemical Tracer Method of Measuring Residual Oil Saturation”, published in October 1980, downloadable from internet, discloses the technique of the SWCTT. All these documents disclose the use of ester compounds as chemical tracers. During the reaction period, the ester tracers dissolved in water in the oil reservoir are hydrolyzed to form alcohol and acid compounds. The alcohol compounds are the secondary tracers which may be analyzed in the returning water.
Even if this test method has demonstrated its efficiency in several real situations, the SWCTT method cannot be implemented efficiently with oil reservoirs having high salinity and/or high temperature conditions. Indeed, the selection of a suitable tracer is dictated at least by two criteria: the partition coefficient of the tracer between oil and water phase (Kd) and the hydrolysis rate of the tracer (kH). The tracer is selected such that its Kd and kH values are close to the optimal values of Kd and kH, which may be determined as disclosed in the DOE report. However, first, the conditions of salinity and of temperature of the well have an effect on the partition coefficient Kd of the tracer between oil and water phase. If the salinity of the reservoir is high, the partition coefficient Kd of the tracer will generally be higher than the optimal partition coefficient. Secondly, high temperature increases dramatically the hydrolysis of the tracers. If the temperature of the reservoir is high, the hydrolysis rate kH of the tracer will generally be higher than the optimal hydrolysis rate.
Consequently, the test methods currently used are not satisfactory for testing oil reservoirs in high salinity and/or high temperature conditions. The currently used tracers are not suitable for these specific conditions. If they were used, the precision of the measures would not be optimal.
In this context, one object of the present invention is to provide an improved test method which would be efficient in high salinity and/or high temperature conditions. This method may advantageously remedy the drawbacks of tracers according to the prior art. In addition, the method may advantageously use tracers which are economical and harmless. Furthermore, the test method of the invention may be advantageously simple to implement and economical.